Method for evaluating delayed fracture of metal material

ABSTRACT

A method for evaluating the delayed fracture characteristics of a metal material. The method including placing a solution-retaining material impregnated with a solution containing a chloride and having a pH of 3.5 or more on a stress loading part of the metal material, and maintaining a state in which the solution-retaining material is held at a deliquescence humidity of the chloride to thereby corrode the stress loading part.

TECHNICAL FIELD

The application relates to a delayed fracture evaluation method forevaluating the delayed fracture characteristics of stress loading partssuch as bent portions of metal materials used in a wet corrosiveenvironment.

BACKGROUND

When hydrogen enters automotive parts materials with increased strength,a phenomenon called “delayed fracture” occurs in which mechanicalproperties such as elongation deteriorate. It is known that the delayedfracture of a material is induced by an increase in the amount ofhydrogen entering the material and that the sensitivity to delayedfracture increases as the strength of the material and the stressapplied thereto increase. In particular, to produce automotive parts, asteel sheet used as a raw material (blank sheet) is generally subjectedto bending, stretching, etc. using a press machine or a sheet metalmachine in many cases. Here, in stress loading parts to which stress hasbeen applied during the bending, stretching, etc., delayed fracturetends to be a problem.

For evaluating delayed fracture, an acid immersion test, a cathodiccharging test, a corrosion test, etc. have conventionally been used tointroduce hydrogen. Non Patent Literature 1 describes a technique forevaluating delayed fracture characteristics by immersing a material inan aqueous hydrochloric acid solution to introduce hydrogen into thematerial. Patent Literature 1 describes a technique for simplyevaluating delayed fracture characteristics by introducing hydrogen intoa steel sheet with stress applied thereto by a cathodic charging test.Patent Literature 2 describes a technique for simply evaluating delayedfracture characteristics of a metal material used in an atmosphericcorrosive environment. Specifically, in this technique, a dry-wetprocess that occurs in an atmospheric corrosive environment during dayand night is simulated to evaluate the delayed fracture characteristicsof the material subjected to corrosion. Patent Literature 3 describes anelectrochemical corrosion evaluation method that uses a water-containingmaterial with attention given to the shape of an automotive part used asa test object. Specifically, in this laboratory corrosion resistanceevaluation method, a water-containing muddy material is used to reducethe influence of surface irregularities.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.    2005-134152-   PTL 2: Japanese Unexamined Patent Application Publication No.    2016-180658-   PTL 3: Japanese Unexamined Patent Application Publication No.    2019-32173

Non Patent Literature

-   NPL 1: Tetsu-to-Hagane (The journal of the Iron and Steel Institute    of Japan), Vol. 79, No. 2, Page 227-232

SUMMARY Technical Problem

As described above, delayed fracture often occurs in metal materialssubjected to bending, bulging, etc., and it is necessary to evaluate thedelayed fracture of the stress loading parts of these worked metalmaterials. The evaluation methods described in Non Patent Literature 1,Patent Literature 1, and Patent Literature 3 can also be used to testthe stress loading parts. However, in the tests in Non Patent Literature1 and Patent Literature 1, a wet corrosive environment in which testmaterials are in a wet state because of snowfall, rainfall, or splashingis not taken into consideration, and it is therefore difficult todetermine the delayed fracture characteristics in an actual environment.

Patent Literature 2 is premised on the direct evaluation of the delayedfracture characteristics of materials because the test is performed inan environment in which a dry-wet behavior occurs during day and night.However, automobiles are used in various environments, and it is notassumed that this test is used in snowy areas and in an environment inwhich a flood occurs. The object of Patent Literature 3 is penetrationinto a surface-treated film using a water-containing material anddiffers from the idea of the disclosed embodiments, i.e., maintenance ofthe thickness of a liquid film.

The disclosed embodiments have been made in view of the foregoingcircumstances, and it is an object to provide a method for evaluatingdelayed fracture of metal materials. The method can accurately evaluatethe delayed fracture characteristics of stress loading parts of themetal materials in a wet corrosive environment such as a snowfall,rainfall, or splashing environment.

Solution to Problem

The present inventors have conducted studies to achieve the foregoingobject, and the disclosed embodiments are summarized as follows.

[1] A method for evaluating delayed fracture characteristics of a stressloading part of a metal material, the method including: placing asolution-retaining material impregnated with a solution containing achloride and having a pH of 3.5 or more on the stress loading part; andmaintaining a state in which the solution-retaining material is held ata deliquescence humidity of the chloride to thereby corrode the stressloading part.

[2] The method for evaluating the delayed fracture characteristics ofthe metal material according to [1], wherein the corrosion is allowed tocontinue while a thickness of a liquid film of the solution ismaintained at from 10 μm to 2500 μm inclusive.

[3] The method for evaluating the delayed fracture characteristics ofthe metal material according to [1] or [2], wherein the corrosion isperformed at a test temperature of -50 to 60° C.

[4] The method for evaluating the delayed fracture characteristics ofthe metal material according to any of [1] to [3], wherein, after thesolution containing the chloride and having a pH of 3.5 or more issupplied to the stress loading part, the solution-retaining material isplaced on the stress loading part.

[5] The method for evaluating the delayed fracture characteristics ofthe metal material according to [4], wherein the solution is supplied byone of immersion for shorter than 15 minutes, atomizing, showering, anddropwise addition.

[6] The method for evaluating the delayed fracture characteristics ofthe metal material according to any of [1] to [5], wherein the metalmaterial is a steel sheet having a tensile stress of 1180 MPa or more.

Advantageous Effects

According to the disclosed embodiments, the delayed fracturecharacteristics of a stress loading part of a metal material in a wetcorrosive environment such as a snowfall, rainfall, or splashingenvironment can be evaluated accurately.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic illustration showing an example of a test specimenused for a delayed fracture evaluation method.

FIG. 2 A schematic illustration showing an example of the test specimenused for the delayed fracture evaluation method.

DETAILED DESCRIPTION

The disclosed embodiments will now be described. The followingdescription of the preferred embodiment is merely exemplary in natureand is not intended to limit its application or use. The presentinventors have evaluated the delayed fracture characteristics of highstrength steel sheets used for automobiles in various actualenvironments. Then the inventors have clarified that delayed fracture islikely to occur particularly in an environment in which a snow meltingsalt is scattered. This is because snow and mud swirled by runningautomobiles and adhering to the surfaces of the steel sheets maintainthe wet state of the surfaces thereof, so that the corrosive conditionsare severest.

When the surface of a metal material and its stress loading part towhich, for example, bending stress is applied are placed in the samecorrosive environment, the wet state is maintained also at the stressloading part because of adhesion of snow and mud thereto although it hasa curved shape, and it has been found that this environment is severefor delayed fracture. Thus, the inventors have found that it isnecessary to perform a test for evaluating delayed fracturecharacteristics of a stress loading part of a metal material in a wetcorrosive environment such as a snowfall, rainfall, or splashingenvironment in addition to the evaluation of the delayed fracturecharacteristics of the surface of the metal material.

The inventors have also found that the wet state is maintained not onlyon the surface of the metal material but also at the stress loading partdue to the adhesion of snow and mud. It has been found based on theseresults that it is necessary to maintain a uniform wet state also in thestress loading part in the evaluation test also.

Accordingly, the method for evaluating the delayed fracture of a metalmaterial according to the disclosed embodiments is performed by: placinga solution-retaining material impregnated with a solution containing achloride and having a pH of 3.5 or more on a stress loading part of themetal material; and maintaining a state in which the solution-retainingmaterial is held at a deliquescence humidity of the chloride to therebycorrode the stress loading part. In the following exemplification of theembodiment, the delayed fracture evaluation method performed includes:(1) a supplying step of supplying the solution containing chloride ionsand having a pH of 3.5 or more to the stress loading part of the metalmaterial; and (2) a corroding step of corroding the metal material byplacing the stress loading part with the solution supplied thereto in anenvironment with a test temperature of -50 to 60° C. and a humidityequal to or higher than the deliquescence humidity of the chloride whilethe liquid film of the solution is maintained.

First, to specifically evaluate the delayed fracture characteristics, itis necessary that the metal material have a stress loading part.Examples of a method for working the metal material include bending,bulging, and stretching. Examples of a method for applying stress to themetal material include a method in which a bolt is used to fix the metalmaterial with the stress applied thereto and a method in which residualstress present after working is used for evaluation. In an example to beshown, bending stress is applied to the stress loading part. However,axial load stress due to a tensile or compression load, torsionalstress, etc. may be applied.

In particular, the metal material used for the evaluation is a steelmaterial such as a steel sheet having a tensile stress TS of 1180 MPa ormore, but this is not a limitation. Other metal materials such as Ti andAl may be used. Moreover, the metal materials include coated metalmaterials.

(1) Supplying step

The supplying step is the step of supplying the solution containing achloride and having a pH of 3.5 or more to the stress loading part ofthe metal material. If the pH of the solution is lower than 3.5,dissolution of the metal material is facilitated. As the metal materialdissolves in the solution, hydrogen ions in the solution are reduced, sothat intrusion of hydrogen into the metal material is facilitated.Specifically, if the pH of the solution is lower than 3.5, theenvironment of the test is severer than the actual environments, and thedelayed fracture characteristics in the actual environments cannot betested accurately. Therefore, the pH of the solution used is 3.5 ormore. In the actual corrosive environments, the solution is neutral, andthe pH of the solution is preferably 5 to 9.

The chloride is a general cause for the actual corrosive environmentsand is therefore contained in the solution. The stress loading part ofthe metal material may be simply covered with snow that contains nochloride. However, even when the stress loading part of the metalmaterial is covered with snow, the stress loading part may be affectedby a snow melting salt. Therefore, the above solution contains thechloride. In this case, the chloride is contained in the solution aschloride ions.

The chloride ions mean Cl ions in Cl ion-containing salts such as NaCl,MgCl₂, and CaCl₂, and the solution contains at least one of the abovechloride components. Water is used as a solvent, and the solution maycontain only one chloride such as NaCl, MgCl₂, or CaCl₂. The solutionmay contain a plurality of chlorides and may contain a component otherthan the chlorides. Examples of the component other than the chloridesinclude, but not limited to, sulfides and nitric acid compoundscontained in the environment and urea mixed in the snow melting agentand scattered. Considering the actual environments, the solution causedto adhere to the metal material is preferably a solution composed mainlyof NaCl, for example, salt water.

The solution is a liquid state mixture of two or more materials. Thesolution used may be, for example, an aqueous solution containing aliquid component composed of water as a solvent and any of theabove-described chlorides as a solute. In particular, in the solutionused for the delayed fracture test, the chloride accounts for 50 to 100%by weight of the solutes and preferably 70% by weight or more. Byincreasing the weight ratio of the chloride as described above, thedeliquescence humidity described later can be easily controlled. Nolimitation is imposed on the concentrations of the solutes and thesolvent in terms of % by weight so long as no operational inconvenienceoccurs. For example, when the volume of NaCl is equal to or more thanone half of the volume of the solution, the amount of NaCl is largerthan its saturation amount, so that precipitation occurs. In this case,the solution cannot be supplied uniformly and is therefore not suitablefor the disclosed embodiments.

No particular limitation is imposed on the method for supplying thesolution to the stress loading part of the metal material. Specificexamples include: an immersion method in which a test specimen isimmersed in the solution (for shorter than 15 minutes and preferablyshorter than 10 minutes) and removed therefrom to cause the solution toadhere to the surface of the test specimen; a method in which thesolution is applied to the metal material by spraying or showering; amethod in which the solution is caused to adhere by atomizing; and amethod in which a prescribed amount of the solution is added dropwiseusing a pipet. If the immersion is performed for longer than 15 minutesto cause the solution to adhere, corrosion proceeds in the solution.This differs from the corrosion mode of the disclosed embodiments and isnot suitable. The spraying means that droplets of the solution of 50 μmor larger are injected, and the atomizing means that droplets of thesolution of smaller than 50 μm are injected.

In the supplying step, the solution may be supplied by placing thesolution-retaining material such as gauze impregnated with the solutionto be used in the corroding step on the stress loading part. In thiscase, the corroding step described below is performed using thesolution-retaining material placed in the supplying step. In thismanner, the actual corrosive environment in which the state remainsunchanged from the supplying step to the corroding step can bereproduced, and the accuracy of the test can be improved.

(2) Corroding step

In the corroding step, the solution-retaining material impregnated withthe solution is disposed on the stress loading part of the metalmaterial, and the state in which the solution-retaining material is heldat the deliquescence humidity of the chloride is maintained to corrodethe stress loading part of the metal material.

In the corroding step, it is necessary to continue holding the solutionon the stress loading part of the metal material, and the thickness ofthe film of the solution is an important factor that has a significantinfluence on the delayed fracture characteristics. Using differentliquid film thicknesses, the delayed fracture characteristics at theseliquid film thicknesses were actually evaluated. It was found that thedelayed fracture characteristics can be evaluated when at least theliquid film is present on the stress loading part (the liquid film isthicker than 0 μm). The liquid film thickness is obtained from theresults of conversion of a value obtained by an ACM sensor (manufacturedby Syrinx Inc.) to the liquid film thickness. In particular, it ispreferable to maintain the liquid film thickness steadily at from 10 μmto 2500 μm. If the liquid film thickness is less than 10 μm, the liquidfilm formed is not sufficient, and the degree of corrosion is lower thanthat in a test in an actual environment. In this case, although acritical load stress coincides with that in the actual environment, thecorrosion mode (corrosion type) may differ from that in the actualenvironment. Similarly, if the liquid film thickness exceeds 2500 μm,the liquid film thickness is excessively large. In this case also,although the critical load stress obtained coincides with that in theactual environment, the corrosion mode (corrosion type) may differ fromthat in the actual environment. Therefore, the liquid film thickness ispreferably from 10 μm 2500 μm. The liquid film thickness in thecorroding step is controlled mainly by the amount of the solutionsupplied in the supplying step described above and the amount of thechloride described later.

No particular limitation is imposed on the test temperature in thecorroding step, but the test is performed in an environment with a testtemperature of, for example, -50 to 60° C. This test temperature wasdefined based on measurements in actual environments. It has been foundthat, in some environments in which a snow melting salt is used, thetemperature can reach −50° C. When an automobile is parked in directsunlight, the temperature of body parts of the automobile can reach ashigh as 60° C. Therefore, the temperature range in the disclosedembodiments is set to -50 to 60° C. Considering the range of the testtemperature achievable in a general purpose apparatus, the testtemperature is preferably -20 to 40° C.

In the corroding step, the relative humidity of the test environment isone of the significant factors, and it has been found from variousexperiments that it is necessary to steadily maintain the state in whichthe stress loading part wet with the solution is not dried in thecorroding step. The wet state in which the stress loading part is notdried means that one continuous liquid film (the liquid film thicknessis preferably equal to or more than 10 μm) is formed on the stressloading part of the metal material without discontinuities. Whendiscontinuities occur in the liquid film, the amount of the chloride isnonuniform in the test area. This is significantly different from thestate in actual environments and is not preferred. To form onecontinuous liquid film without discontinuities, the wet state must bemaintained with only minor fluctuations.

Therefore, the corroding step is performed in an environment with ahumidity equal to or higher than the deliquescence humidity of thechloride. The deliquescence humidity is the humidity at which moistureabsorption by the chloride (deliquescence of the chloride) causes thesurface of the test specimen to be wet. In this manner, theabove-described state in which the solution on the stress loading partis not dried can be maintained. The deliquescence humidity is determinedby the type of chloride supplied to the stress loading part, i.e., thetype of chloride in the solution. For example, when a salt composedmainly of NaCl is used, the relative humidity is set to 75% RH orhigher. When a salt composed mainly of MgCl₂ is used, the relativehumidity is set to 33% RH or higher. When a salt composed mainly of KClis used, the relative humidity is set to 84% RH or higher. Theenvironment is controlled uniformly such that the humidity does notfluctuate. Allowable fluctuations in the liquid film thickness in thecorroding step are ±10% of the set value thereof. Fluctuations exceedingthese values are not suitable because unevenness of the evaluationresults occurs.

As described above, the amount of the chloride in the solution and therelative humidity of the test environment are set such that the liquidfilm thickness is maintained without discontinuities. Specifically, theliquid film thickness is determined by the relative humidity of the testenvironment and the amount of the chloride. It is difficult to reproducean actual environment either when the amount of the chloride isexcessively large or excessively small. The amount of the chloride thatallows the above-described liquid film thickness to be maintained ispreferably 1000 to 200000 mg/m².

For example, in an environment with a temperature of 25° C. and ahumidity of 95% RH, the liquid film thickness is about 10 μm when theamount of the chloride is 0.1 g/m² and is about 100 μm when the amountof the chloride is 1 g/m². Based on consideration of the liquid filmthickness from the viewpoint of controlling the amount of the chloridesupplied and the relative humidity, the liquid film thickness in thecorroding step is preferably 40 to 1500 μm. The relative humidity of thetest environment is more preferably 90% RH so that the results ofexperiments are not affected even when the relative humidity of the testenvironment fluctuates +5% RH.

Moreover, in the corroding step, the solution-retaining materialimpregnated with the solution is placed on the stress loading part ofthe metal material such that the state in which the solution supplied inthe supplying step is retained is maintained. In this manner, the wetstate of the stress loading part of the metal material can be maintainedreliably. The solution-retaining material may be a material such ascotton gauze or mud that has void spaces thereinside so that thesolution retained by capillarity can be maintained. In order not toinhibit corrosion, the solution-retaining material is preferably amaterial that can transmit oxygen. To steadily maintain the wet state ofthe stress loading part of the metal material, it is preferable not tomove the solution-retaining material during the test. The supplying stepmay be performed only once at the beginning, or the supplying step andthe corroding step may be repeated. When the supplying step and thecorroding step are repeated, it is preferable to perform the corrodingstep for 100 hours or longer.

EXAMPLES Example 1

First, whether the delayed fracture test method of the disclosedembodiments can simulate the actual corrosion was examined in Example 1below. First, steel types A, B, C, and D having compositions shown inTable 1 were used as metal materials serving as test specimens.

TABLE 1 C Si Mn YS TS (mass %) (mass %) (mass %) (MPa) (MPa) Steel typeA 0.45 0.20 0.73 1370 1500 Steel type B 0.21 0.33 1.60 1310 1520 Steeltype C 0.34 0.20 1.20 1350 1490 Steel type D 0.33 0.08 2.00 1290 1510

FIG. 1 is a schematic illustration showing an example of a test specimenused for the evaluation of delayed fracture. As shown in FIG. 1 , a 1.4mm-thick steel sheet formed from steel type A, B, C, or D was sheared toa width of 35 mm x a length of 100 mm; to remove residual stressgenerated during shearing, the opposite edges of the specimen wereground until its width became 30 mm; and holes for bolts were formed atpositions spaced apart from an evaluation portion to thereby produce atest specimen 1.

This steel sheet for the test was immersed in toluene, subjected toultrasonic cleaning for 5 minutes, and then bent 180°, and the testspecimen 1 in a springback state was restrained by a bolt BN and a nutNN to thereby complete the test specimen 1. The test specimen 1 for theevaluation of delayed fracture has a stress loading part 2 composed of abent portion with a bend radius R=7 mm, and a stress was supplied to thestress loading part 2 by adjusting the tightening width of the bolt BNand the nut NN. The larger the degree of tightening, the larger the loadstress, and the severer the conditions. The stress applied to the stressloading part 2 by tightening is referred to as tightening stress. In thepresent Example, five levels of tightening stress including 800, 1000,1200, 1400, and 1600 MPa were used. The tightening stress was determinedby estimating the tightening width using CAE analysis based on the SScurve of the material used.

For the test specimens 1 using steel types A and B, an actualenvironment test was performed in a region in which a snow melting saltwas actually scattered, and also a test using the delayed fractureevaluation method for metal materials according to the disclosedembodiments was performed. For the test specimens 1 using steel types Cand D, the test using the delayed fracture evaluation method for metalmaterials described later was performed.

<Actual Environment Test>

In the actual environment test, each test specimen 1 was placed in alower portion of a mobile object that traveled every day on roads with asnow melting salt scattered thereon and collected 60 days after thestart of the test. The reason that the specimens were placed in thelower portion of the mobile object is that the lower portion is affectedby the snow melting salt. The results are shown in Table 2 below. Theminimum load stress at which cracking occurred during the test wasregarded as a border for the occurrence of cracking and defined as acritical load stress in the actual environment.

TABLE 2 Applied Cracking Critical Steel load stress evaluation loadstress type (MPa) result (MPa) A 800 ◯ 1400 1000 ◯ 1200 ◯ 1400 X 1600 XB 800 ◯ 1200 1000 ◯ 1200 X 1400 X 1600 X

In Table 2, when a crack of 1 mm or longer was found in a test specimen1, the condition of this test specimen was judged as cracked (symbol:x). When a crack of shorter than 1 mm or no cracking was found in a testspecimen 1, the condition of this test specimen 1 was judged as notcracked (symbol: 0). Among the load stresses applied to cracked testspecimens 1, the smallest load stress was defined as the critical loadstress. The critical load stress of the steel A was 1400 MPa, and thecritical load stress of the steel B was 1200 MPa.

<Test for Delayed Fracture Characteristics>

Next, as shown in FIG. 2 , the same test specimens 1 as those in theactual environment test described above were used and asolution-retaining member 10 was placed on the stress loading part 2 ofeach specimen in the corroding step to perform the delayed fractureevaluation method for metal materials described above. In this test, themaximum test period was 60 days, and the minimum load stress at whichcracking occurred during the test period was regarded as the border forthe occurrence of cracking and defined as the critical load stress. Thecritical load stress and the mode of corrosion (corrosion type) obtainedin the delayed fracture characteristic test were compared with thecritical load stress and the mode of corrosion (corrosion type) obtainedin the actual environment test to determine whether the delayed fracturecharacteristic test is appropriate. The allowable test range of therelative humidity in the air atmosphere was the set value ±5%.

The test conditions of the delayed fracture evaluation test and theresults are shown in Table 3.

TABLE 3 Estimated liquid Relative Compati- film humidity Amount of TestCritical bility thickness of the test chloride temper- Solution- loadwith Test Steel (compu- environment supplied/ Solution ature/ retainingSupplying stress/ actual number type tation)/μm % RH mg/m² supplied ° C.pH material method MPa environment 1 A  0 70 10000 Aqueous 0 7.0 CottonSpraying 1600 x Comparative NaCl gauze or more Example solution 2 A  585 1000 Aqueous 0 7.0 Cotton Spraying 1400 B Example NaCl gauze solution3 A  9 95 700 Aqueous 0 7.0 Cotton Spraying 1400 B Example NaCl gauzesolution 4 A  30 75 10000 Aqueous 0 7.0 Cotton Spraying 1400 A ExampleNaCl gauze solution 5 A  40 95 3000 Aqueous 0 7.0 Cotton Spraying 1400 AExample NaCl gauze solution 6 A  40 95 3000 Aqueous 0 7.0 CottonDropwise 1400 A Example NaCl gauze addition solution 7 A  40 95 3000Aqueous 0 7.0 Mud Spraying 1400 A Example NaCl solution 8 A 125 95 10000Aqueous −60 7.0 Cotton Spraying 1400 B Example NaCl gauze solution 9 A125 95 10000 Aqueous −50 7.0 Cotton Spraying 1400 A Example NaCl gauzesolution 10 A 125 95 10000 Aqueous −20 7.0 Cotton Spraying 1400 AExample NaCl gauze solution 11 A 125 95 10000 Aqueous 0 3.0 CottonSpraying 1000 x Comparative NaCl gauze Example solution 12 A 125 9510000 Aqueous 0 3.5 Cotton Spraying 1400 A Example NaCl gauze solution13 A 125 95 10000 Aqueous 0 7.0 Cotton Spraying 1400 A Example NaClgauze solution 14 A 125 95 10000 Aqueous 0 7.0 Cotton Dropwise 1400 AExample NaCl gauze addition solution 15 A 125 95 10000 Aqueous 0 7.0Cotton Immersion 1400 A Example NaCl gauze (10 min) solution 16 A 125 9510000 Aqueous 0 7.0 Mud Spraying 1400 A Example NaCl solution 17 A 12595 10000 Aqueous 0 7.0 None Spraying 1600 x Comparative NaCl or moreExample solution 18 A 125 95 10000 Aqueous 0 10.0  Cotton Spraying 1400A Example NaCl gauze solution 19 A 125 95 10000 Aqueous 30 7.0 CottonSpraying 1400 A Example NaCl gauze solution 20 A 125 95 10000 Aqueous 607.0 Cotton Spraying 1400 A Example NaCl gauze solution 21 A 125 95 10000Aqueous 70 7.0 Cotton Spraying 1400 B Example NaCl gauze solution 22 A125 95 10000 Sea salt 0 7.0 Cotton Spraying 1400 A Example (NaCl + gauzeMgCl₂) 23 A 2500  95 200000 Aqueous 0 7.0 Cotton Spraying 1400 A ExampleNaCl gauze solution 24 A 2500  95 200000 Aqueous 0 7.0 Cotton Dropwise1400 A Example NaCl gauze addition solution 25 A 2500  95 200000 Aqueous0 7.0 Mud Spraying 1400 A Example NaCl solution 26 A 3000  95 250000Aqueous 0 7.0 Cotton Spraying 1400 B Example NaCl gauze solution 27 A —95 — Ion 0 7.0 Cotton Spraying 1600 x Comparative exchanged gauze ormore Example water 28 B  0 70 10000 Aqueous 0 7.0 Cotton Spraying 1600 xComparative NaCl gauze or more Example solution 29 B  5 85 1000 Aqueous0 7.0 Cotton Spraying 1200 B Example NaCl gauze solution 30 B  9 95 700Aqueous 0 7.0 Cotton Spraying 1200 B Example NaCl gauze solution 31 B 30 75 10000 Aqueous 0 7.0 Cotton Spraying 1200 A Example NaCl gauzesolution 32 B  40 95 3000 Aqueous 0 7.0 Cotton Spraying 1200 A ExampleNaCl gauze solution 33 B  40 95 3000 Aqueous 0 7.0 Cotton Dropwise 1200A Example NaCl gauze addition solution 34 B  40 95 3000 Aqueous 0 7.0Mud Spraying 1200 A Example NaCl solution 35 B 125 95 10000 Aqueous −607.0 Cotton Spraying 1200 B Example NaCl gauze solution 36 B 125 95 10000Aqueous −50 7.0 Cotton Spraying 1200 A Example NaCl gauze solution 37 B125 95 10000 Aqueous −20 7.0 Cotton Spraying 1200 A Example NaCl gauzesolution 38 B 125 95 10000 Aqueous 0 3.0 Cotton Spraying 800 xComparative NaCl gauze Example solution 39 B 125 95 10000 Aqueous 0 3.5Cotton Spraying 1200 A Example NaCl gauze solution 40 B 125 95 10000Aqueous 0 7.0 Cotton Spraying 1200 A Example NaCl gauze solution 41 B125 95 10000 Aqueous 0 7.0 Cotton Dropwise 1200 A Example NaCl gauzeaddition solution 42 B 125 95 10000 Aqueous 0 7.0 Cotton Immersion 1200A Example NaCl gauze (10 min) solution 43 B 125 95 10000 Aqueous 0 7.0Mud Spraying 1200 A Example NaCl solution 44 B 125 95 10000 Aqueous 07.0 None Spraying 1600 x Comparative NaCl or more Example solution 45 B125 95 10000 Aqueous 0 10.0  Cotton Spraying 1200 A Example NaCl gauzesolution 46 B 125 95 10000 Aqueous 30 7.0 Cotton Spraying 1200 A ExampleNaCl gauze solution 47 B 125 95 10000 Aqueous 60 7.0 Cotton Spraying1200 A Example NaCl gauze solution 48 B 125 95 10000 Aqueous 70 7.0Cotton Spraying 1200 B Example NaCl gauze solution 49 B 125 95 10000 Seasalt 0 7.0 Cotton Spraying 1200 A Example (NaCl + gauze MgCl₂) 50 B2500  95 200000 Aqueous 0 7.0 Cotton Spraying 1200 A Example NaCl gauzesolution 51 B 2500  95 200000 Aqueous 0 7.0 Cotton Dropwise 1200 AExample NaCl gauze addition solution 52 B 2500  95 200000 Aqueous 0 7.0Mud Spraying 1200 A Example NaCl solution 53 B 3000  95 250000 Aqueous 07.0 Cotton Spraying 1200 B Example NaCl gauze solution 54 B — 95 — Ion 07.0 Cotton Spraying 1600 x Comparative exchanged gauze or more Examplewater A = both the critical load stress and the corrosion mode coincidedwith those in the actual environment B = only the critical load stresscoincided with that in the actual environment x = either did notcoincide with those in the actual environment

In Examples in Table 3, the test was performed under the test conditionsdescribed in the disclosed embodiments. In Comparative Examples, atleast one of the test conditions was outside the corresponding range inthe embodiment, and conditions outside the numerical ranges in thedisclosed embodiments are underlined. When no cracking was found in allthe test specimens under certain conditions, 1600 MPa or more was placedin the critical load stress column for these conditions. The symbol “x(poor)” was given to the conditions under which the results did notcoincide with those in the actual environment test (ComparativeExamples). The symbol “B” was given to the conditions under which theresults coincided with those in the actual environment test, and thesymbol “A” was given to more preferred results. Specifically, asdescribed above, the compatibility with the actual environment wasevaluated in terms of the critical load stress and the corrosion mode(corrosion type). When either of the critical load stress or thecorrosion mode (corrosion type) coincided with those in the actualenvironment, “A” was assigned. When the critical load stress coincidedbut the corrosion mode was different, B″ was assigned because thedelayed fracture evaluation test itself was valid. When either thecritical load stress or the corrosion mode (corrosion type) did notcoincide with those in the actual environment, the test performed wasnot suitable as the delayed fracture evaluation test, and the symbol “x”was given.

Nos. 3, 5, 13, 23, and 26 are Examples using steel type A. In theseExamples, conditions other than the amount of the chloride were thesame, and the amount of the chloride was changed to change the liquidfilm thickness. In No. 3, since the amount of the chloride supplied wassmall, the liquid film was not formed sufficiently, and the degree ofcorrosion was less than that in the actual environment test. Therefore,although the cracking evaluation result (critical load stress) coincidedwith that in the actual environment, the environment was milder. In No.26, the amount of the chloride was large, and the liquid film thicknesswas excessively large. Therefore, although the critical load stresscoincided with that in the actual environment, the corrosion mode wasdifferent from that in the actual environment. Nos. 30, 32, 40, 50, and53 are Examples using steel type B, and a similar tendency was found.

Nos. 1, 2, 4, 13, 28 to 29, 31, and 40 are Examples and ComparativeExamples using steel type A or B. In these Examples and ComparativeExamples, the relative humidity of the test environment and the amountof chloride were changed to change the liquid film thickness. Thethickness of the liquid film formed is determined from the relationbetween the relative humidity of the test environment and the amount ofthe chloride. In Nos. 4 and 31, the humidity was 75% RH, and the amountof the chloride was 10000 mg/m². Therefore, the state in which theliquid film is present without discontinuities (liquid film thickness:10 μm or more) can be maintained. In Nos. 1 and 28 that are ComparativeExamples, the relative humidity of the test environment was low. Thisrelative humidity of the test environment is lower than the relativehumidity at which the chloride contained in the solution absorbsmoisture. Therefore, substantially no liquid film was formed, andcorrosion did not proceed. This environment differs from the actualenvironment. In Examples in Nos. 13 and 40, the relative humidity of thetest environment was 90% or more. In these Examples, the water filmthickness was in the preferred range, and the corrosion state wassimilar to that in the actual environment, so that preferred resultswere obtained.

Nos. 13, 22, 27, 40, 49, and 54 are Examples and Comparative Examples inwhich different types of solutions were used. In Examples in Nos. 13,22, 40, and 49, a solution containing a chloride(s) was used, and theresults coincided with those in the actual environment. In ComparativeExamples Nos. 27 and 54, a solution containing no chloride ions wasused. Since no chloride ions were contained, the liquid film formed wasnot uniform, and the results did not coincided with those in the actualenvironment.

Nos. 8 to 10, 13, 19 to 21, 35 to 37, 40, 46 to 48 are Examples in whichdifferent test temperatures were used. In Examples in Nos. 9, 10, 13,19, 20, 36, 37, 40, 46, and 47, the results coincided well with those inthe actual environment. However, in Nos. 8 and 35, since the temperaturewas excessively low, the solution was frozen, and the corrosion did notproceed beyond a certain point. Therefore, although the crackingevaluation result (critical load stress) coincided with that in theactual environment, the corrosion mode did not coincide. When thetemperature was excessively high as in Nos. 21 and 48, the degree ofcorrosion was higher than that in the actual environment. In this case,although the cracking evaluation result (critical load stress) coincidedwith that in the actual environment, the corrosion mode did notcoincide.

Nos. 11 to 13, 18, 38 to 40, and 45 are Examples and ComparativeExamples in which different pHs were used. The results in Examples Nos.12 to 13, 18, 39 to 40, and 45 coincided well with those in the actualenvironment. In Comparative Examples in Nos. 11 and 38, the results wereinferior to those in the actual environment. Since the pH of thesolution was low, dissolution of iron was facilitated, and hydrogen ionsin the solution were reduced due to the dissolution of iron, so thatintrusion of hydrogen into the steel was facilitated. Therefore, theenvironments of the comparative examples were severer than the actualenvironment and the results thereof did not coincide with those in theactual environment.

Nos. 5, 7, 13, 16 to 17, 23, 25, 32, 34, 40, 43 to 44, 50, and 52 areExamples and Comparative Examples in which different solution-retainingmaterials were used. Nos. 5, 7, 13, 16, 23, 25, 32, 34, 40, 43, 50, and52 are Examples in which different solution-retaining materials wereused. In each of these Examples, the solution-retaining materialdisposed was impregnated with the solution, and the liquid filmthickness could be maintained, so that the results coincided with thosein the actual environment. In Nos. 17 and 44, no material having theability to retain water was disposed, so that the solution could not beretained. Therefore, the state of progress of corrosion differs fromthat in the actual environment, and the results were different fromthose in the actual environment.

Nos. 5 to 6, 13 to 15, 23 to 24, 32, 33, 40 to 42, 50 to 51 are Examplesin which different solution supply methods were used. In Examples inNos. 5 to 6, 13 to 15, 23 to 24, 32, 33, 40 to 42, 50 to 51, althoughdifferent supply methods were used, the results coincided with those inthe actual environment so long as the conditions for the liquid filmthickness were satisfied.

Example 2

Immersion in hydrochloric acid is a conventional delayed fractureevaluation method. With this method, only comparison between materialsunder the same conditions is possible. Therefore, whether or not thepresent evaluation can determine the limit of use of a material itselfwas verified. Specifically, the delayed fracture characteristics of thesteel C and the steel D were evaluated from their critical load stress.The results are shown in Table 4.

TABLE 4 Relative Estimated Humidity liquid film of the Amount ofCritical thickness test chloride Solution- load Steel (compu- environ-supplied/ Solution retaining Supplying stress/ type tation)/μm mentmg/m² supplied pH material method MPa C 125 95% RH 10000 Aqueous 7.0Cotton Spraying 1600 NaCl gauze solution D 125 95% RH 10000 Aqueous 7.0Cotton Spraying 1000 NaCl gauze solution

As shown in Table 4, with the steel C, only a test specimen with a loadstress of 1600 MPa, which was the highest load stress, was cracked. Asis clear from the present Example, since the critical load stress of thesteel C is large, its delayed fracture characteristics are good.However, with the steel D, the critical load stress was 1000 MPa, andcracking occurred at stresses lower than the YS. According to thepresent test, the critical load stress of the steel D was found to below, and it could be determined that the delayed fracturecharacteristics were low. As described above, by using the technique ofthe disclosed embodiments, the limit of use of a material itself can beevaluated.

The disclosed embodiments are not intended to be limited to theabove-described embodiment, and various modifications can be made. Forexample, a metal material to be evaluated is generally a steel materialsuch as a steel sheet, but this is not a limitation. Metal materialssuch as Ti and Al may be used. The delayed fracture characteristicevaluation method of the disclosed embodiments can evaluate the delayedfracture characteristics of metal materials accurately. Therefore, metalmaterials (particularly steel materials such as steel sheets) selectedand evaluated by the evaluation method have good delayed fracturecharacteristics.

1. A method for evaluating delayed fracture characteristics of a stressloading part of a metal material, the method comprising: placing asolution-retaining material impregnated with a solution containing achloride and having a pH of 3.5 or more on the stress loading part; andcorroding the stress loading part by maintaining a state in which thesolution-retaining material is held at a deliquescence humidity of thechloride.
 2. The method for evaluating the delayed fracturecharacteristics of the metal material according to claim 1, wherein thecorrosion is continued while a thickness of a liquid film of thesolution is maintained in a range of 10 μm to 2500 μm.
 3. The method forevaluating the delayed fracture characteristics of the metal materialaccording to claim 1, wherein the corrosion is performed at a testtemperature in a range of −50 to 60° C.
 4. The method for evaluating thedelayed fracture characteristics of the metal material according toclaim 1, wherein, the placing step includes supplying the solutioncontaining the chloride and having the pH of 3.5 or more to the stressloading part before placing the solution-retaining material on thestress loading part.
 5. The method for evaluating the delayed fracturecharacteristics of the metal material according to claim 4, wherein thesolution is supplied by immersion for shorter than 15 minutes,atomizing, showering, or dropwise addition.
 6. The method for evaluatingthe delayed fracture characteristics of the metal material according toclaim 1, wherein the metal material is a steel sheet having a tensilestress of 1180 MPa or more.